The present invention relates to an electric power steering apparatus.
Conventional power steering apparatuses for a vehicle include an electric power steering apparatus (EPS) provided with a motor as a driving source. The EPS has a feature that a flexibility of the layout is high, and an energy consumption is small, in comparison with a hydraulic power steering apparatus. Accordingly, in recent years, the introduction of the EPS has been considered for many kinds of vehicles ranging from compact vehicles to large-sized vehicles.
In the vehicle steering apparatus, one of the most important issues for achieving excellent steering feel is to suppress vibration transmitted through a steering wheel and noise generated by the vibration. In particular, in the EPS that includes a motor as a driving source, torque ripples of an order component (X-th order component) proportional to an integral multiple of a motor rotational angle (mechanical angle) are generated due to electrical and mechanical factors. The torque ripples of such an order component resonate with the mechanical structure of the EPS and the vehicle, so that the vibration and noise generated by the torque ripples of the order component are further increased. Therefore, as disclosed in Japanese Laid-Open Patent Publication Nos. 2002-136043 and 2002-44925, in order to suppress vibration and noise generated by the motor in the EPS, various countermeasures are conventionally taken in accordance with the cause of the vibration and noise.
However, depending on the cause, structural measures might not sufficiently suppress the vibration and noise due to, for example, precision limits. Therefore, Japanese Laid-Open Patent Publication No. 11-313497 discloses an EPS that feeds a power to a motor by executing current feedback control. The EPS is provided with a notch filter, which removes (attenuates) a specific frequency component included in an actual current value supplied to the motor or a control signal generated based on the actual current value. The publication proposes a structure for removing a resonance frequency component that causes the problem using the notch filter to suppress amplification of vibration and noise due to the resonance.
However, some of the torque ripples of the above-mentioned order component have significantly great excitation energy, and torque ripples of such an order component might cause vibration and noise regardless of resonance. In FIG. 4, the darker the shaded part is, the higher the vibration level becomes, and the areas above dashed lines correspond to the torque ripples of order components. Therefore, in the structure for removing the specific resonance frequency component as disclosed in the publication No. 11-313497, the torque ripples of the order components with high excitation energy are not effectively reduced. Thus, there is yet room for improvements in this point.
Accordingly, it is an objective of the present invention to provide an electric power steering apparatus that accurately reduces torque ripples of order components and effectively suppress generation of vibration and noise.
To achieve the above objective, the present invention provides an electric power steering apparatus including a motor as a power source and a steering force assist device, which applies an assist force for assisting a steering operation of a steering wheel in a vehicle steering system to the steering system. The apparatus includes a control device and an attenuating section. The control device controls actuation of the steering force assist device by feeding a power to the motor by executing current feedback control. The attenuating section is provided in the control device and attenuates a specific frequency component included in an actual current value supplied to the motor or a control signal generated by executing the current feedback control. The attenuating section changes the specific frequency component to be attenuated in accordance with the rotational angular velocity of the motor.
The above-mentioned configuration reduces, in a wide rotational angular velocity range, torque ripples of order components, the frequency of which shifts toward a higher frequency in proportion to the increase in the rotational angular velocity. As a result, generation of vibration and noise is effectively suppressed.